The fabrication of memory devices in semiconductor and HDD applications requires the use photolithography processes to define a pattern in a substrate. A photoresist layer on the substrate is patternwise exposed with radiation that passes through a mask (reticle) having opaque and transparent regions. After the exposed photoresist layer is developed to form a pattern therein, the pattern is etch transferred into the substrate. A common mask is a binary mask comprised of an opaque material such as chrome on a transparent substrate that is typically quartz. Light from a source such as a 248 nm or 193 nm excimer laser passes through regions not blocked by chrome on the mask and presents an aerial image on the photoresist layer. An optical reduction system comprised of a plurality of lenses is used to reduce the size of the mask pattern by a factor of 5:1, for example, in the aerial image impinging upon the photoresist surface. The aerial image has high intensity corresponding to light passing through transparent mask regions, and low intensity (dark areas) where light has been blocked by opaque regions thereby producing “exposed” and “unexposed” regions in the photoresist. A small amount of radiation does reach “unexposed” regions, particularly at borders with “exposed” regions because of diffracted light. This condition limits the minimum feature size that is formed or resolved in the photoresist film. Since there is a constant demand to decrease feature size in order to build a higher density of devices per unit area, numerous resolution enhancement techniques have been developed.
The minimum feature size that can be printed in a photoresist film is defined as R=kλ/NA where R is the minimum resolution, k is a constant for the photolithography process, λ is the exposing wavelength, and NA is the numerical aperture of the projection optics in the exposure tool. A combination of lower k through improved process control, lower λ, and higher NA has enabled a steady reduction in technology nodes during the past 20 years from 180 nm to 45 nm and below. Note that k may also be reduced by enhancements in the mask or photolithography process including the use of attenuated masks, off-axis illumination (OAI), optical proximity correction (OPC), and other means to improve contrast between light and dark areas in the aerial image. FIG. 1 depicts a conventional binary mask for printing an array of island shapes where chrome regions 60 are separated by clear quartz regions 61. However, process latitude and resolution are limited because of poor contrast in the aerial image, especially for photoresist feature sizes that are <250 nm. The pitch p is defined as the sum of linewidth s and the adjacent space v.
A commercial photoresist solution is typically comprised of a polymer, casting solvent, and multiple additives including a photosensitive material that is coated on a wafer and baked on a hot plate to remove substantially all of the solvent and leave a photoresist film. The photosensitive component reacts when exposed to light, and generates an acid, which causes either a deprotection mechanism in a positive tone photoresist where the polymer becomes soluble in a developer solution while unexposed regions remain insoluble, or initiates a crosslinking mechanism in a negative tone photoresist where polymers crosslink to render the exposed regions insoluble in developer while unexposed regions are soluble.
The basic idea behind alternating phase shift masks (AltPSM) is to modify the binary reticle so that alternating clear regions will cause light to be phase shifted 180°. Since the intensity of regular phase light and the intensity of reversed phase light cancel each other in the nominally dark areas, image contrast is improved compared with non-phase shifted masks and attenuated phase shifted masks. However, there are a number of problems with AltPSM. When the layout consists of island type features used to fabricate MTJs, light from adjacent areas that is 180° out of phase will interfere destructively and result in phase edges that are unwanted connections between island features in a positive tone photoresist. As shown in FIG. 2, phase edges 71 generally appear as linear sections between the island shapes 70 in a photoresist patterned on a substrate 8 with a conventional AltPSM. Here, the phase edges are aligned vertically (y-axis direction), but may also appear as horizontal linear sections depending on mask design.
To avoid the phase edge issue, one approach is to use a negative tone photoresist. However, negative tone photoresists have drawbacks including swelling during develop, poor step coverage, a toxic stripper requirement, and sensitivity to ambient oxygen. Another possible corrective measure is introducing grated phase shifted features such as 0° to 60°, 60° to 120°, and 120° to 180° in the mask. This approach requires additional steps in reticle fabrication and a complicated mask design, and is not applicable for very tight (small) pitches. Alternatively, a second mask may be used to expose the photoresist so that the phase edges are removed during the development process. This option requires a complicated layout design that is usually not practical.
Current technology does not enable a solution for printing island features having a small pitch less than about 300 nm, and with acceptable critical dimension uniformity of <3 nm along with a suitable process window (exposure latitude and depth of focus) with AltPSM masks in a manufacturing environment. Therefore, a new photolithography process is needed to enable further advances in realizing high density patterns required for semiconductor and magnetic recording devices that rely on island shapes, which are subsequently etch transferred through an underlying stack of layers to form an array of MTJs, for example.